This invention relates to a hydraulic power steering system for automotive vehicles of a type having a servo motor with a differential piston surface area and a control valve provided with a feedback facility.
According to German Patent Publication No. 1,084,589, a power steering mechanism is disclosed which includes a servo motor having an opposed piston surface area ratio of 1:2 and an associated steering control valve controlling two pairs of valve passages through which pressurized fluid is conducted only to one of the pressure chambers of the servo motor acting on the larger of the piston surfaces. The servo motor chamber containing fluid acting on the smaller of the piston surfaces is in constant fluid communication with the power steering pump associated with the steering mechanism. The foregoing power steering arrangement is also provided with feedback means to control the fluid power exerted on the vehicle steering linkage as a function of the effective pressure in the servo motor. The control valve is arranged to be axially displaced when actuated from one side of the steering gear mechanism opposite the side facing the driver from which the steering spindle extends. In the foregoing power steering arrangement, the pump provides a source of constant fluid flow.
The foregoing prior art power steering arrangement is advantageously simple and safe in operation because the valve piston associated with the control valve is positively controlled in both directions of displacement. However, there is a structional disadvantage in such arrangement because of high frictional forces adversely affecting operation of the feedback means which also prevents the control valve from being directly actuated through the steering spindle. Direct control valve actuation through the steering spindle has been recently imposed as a requirement for private automotive vehicles in particular because of steering sensitivity and precision demands.
The referred to disadvantages in the foregoing prior art power steering arrangement have been overcome in a power steering arrangement as disclosed in German Patent Publication No. 2 165 724, or in U.S. Pat. No. 4,051,766 to Strauff by effecting control through the side of the control valve facing the driver. Accordingly, the steering spindle is directly connected to the control valve assembly which utilizes the same dual control valve passage concept for controlling supply of pressurized fluid to both the servo motor and the feedback facility. Furthermore, provision is made to limit development of feedback pressure allowing a further increase in pressure in the servo motor without any noticeable increase in the feedback moment applied to the steering handwheel once the valve actuating moment has reached a certain value. Also, instead of providing a source of constant flow, a source of constant pressure is provided in the latter power steering arrangement. However, the latter power steering arrangement has the disadvantage of requiring feedback surfaces of different sizes. Furthermore, the valve piston of the control valve is not safeguarded against unintentional steering movements in the event it gets stuck.
It is therefore an object of the present invention to provide a power steering system having an unbalanced servo motor of the aforementioned type which does not have the aforementioned disadvantages of the prior art arrangements. An additional object of the present invention is to provide a fluid power steering system having a feedback facility that may be adjusted by a control external to the power steering gear mechanism.